Conventional thermoplastic resins, for example, vinyl chloride resins such as polyvinyl chloride; acrylic resins such as polymethyl methacrylate; aromatic vinyl resins such as polystyrene and styrene-acrylonitrile copolymer; carbonate resins such as polycarbonate; amide resins such as Nylon 6; polyester resins such as polyethylene terephthalate; olefin resins such as polypropylene; and polymer alloys of those resins, for example, alloy of styrene-acrylonitrile copolymer and polycarbonate, alloy of .alpha.-methylstyrene-acrylonitrile copolymer and polyvinyl chloride and alloy of polystyrene and polyphenylene oxide are inherently low in impact resistance. In order to improve impact resistance of those resins and alloys thereof, generally there have been widely employed methods for adding, to rubber particles, graft copolymer particles obtained by graft-copolymerizing various monomers. Though the degree of improvement of impact resistance by the addition of the graft copolymer particles is remarkable, for further improving the impact resistance efficiently, there have been made many proposals of improving graft copolymer particles. The methods disclosed in those proposals are a method of lowering Tg of rubber particles (JP-A-2-1763, JP-A-8-100095), a method of regulating gel content of rubber particles, a method of matching particle size and particle size distribution of rubber particles in graft copolymer particles to those of thermoplastic matrix resin (S. Wu, Polymer Engineering and Science, 30,753 (1990)), a method of adjusting compatibility of graft copolymer particles with thermoplastic matrix resin (JP-A-2-251553), etc.
However improvement by those methods have reached their limits, and it is difficult to improve impact resistance more significantly. Also when an adding amount of graft copolymer particles is increased, there is a problem that other characteristics, for example, processability, weather resistance and economic efficiency are lowered.
Meanwhile crazing and shearing yield are an important factor on improvement of impact resistance of a thermoplastic resin. In order to cause such phenomena, stress concentration in a molded article is inevitable. For that purpose, rubber particles are added. Optimizing a size, shape and softness (Tg and degree of crosslinking of rubber) of rubber particles also has a great effect on the stress concentration, and it is anticipated that making a large cavity in the rubber particle previously has greater influence on the stress concentration ("Impact Resistance of Plastics" by Ikuo Narisawa, pp. 131, 155, published by Siguma Shuppan (1994)). However this proposal is hypothetical, and how it is realized is not disclosed.
In order to realize production of hollow graft copolymer particles, the present inventors have made various studies even with respect to different techniques which are not usually studied, and have found that when a technique for hollowing of particles which is known in the field of paints is applied, hollow graft copolymer particles can be prepared and that when such hollow graft copolymer particles are added to a thermoplastic resin, impact resistance can be further improved. Thus the present invention was completed.